Despite the curative promise of hematopoietic cell transplant (HCT), many AML patients will relapse while others may not have a fully matched donor, an especially acute problem in ethnic minority groups. We aim to overcome these limitations using a novel radioimmunotherapy (RIT) strategy for haploidentical HCT, requiring only one matched HLA haplotype using our preclinical murine syngeneic AML model. We have shown that RIT using 90Y- and 131I-radiolabeled anti-CD45 antibody targets radiation to sites of leukemia to amplify the radiation to targeted tissues while minimizing non-specific toxicities. However, radionuclides used thus far have limitations, for which we propose to improve current AML treatment options by using the higher energy radionuclide astatine-211 (211At) in targeted anti-CD45 RIT in lieu of total body irradiation during haploidentical HCT. This proposal will: Aim 1: To compare the engraftment kinetics, toxicities, and survival observed with 211At-anti-CD45 RIT for haploidentical HCT with that observed using either 90Y-anti-CD45 RIT or standard TBI in a murine leukemia model. Aim 2: To evaluate the microscopic biodistribution of alpha- and beta-emitters in spleen and bone marrow of leukemic mice using novel digital autoradiographic imaging technologies, and estimate the radiation requirements that facilitate disease control and engraftment from haploidentical HCT delivered by 211At- and 90Y-anti-CD45 RIT. Aim 3: To characterize the magnitude and mechanism of cellular damage induced by the alpha- and beta-emitters, 211At and 90Y, when targeted to leukemic cells by anti-CD45 RIT in a disseminated murine AML model. We anticipate these preclinical studies to define a novel haploidentical HCT conditioning regimen using 211At- anti-CD45 RIT without TBI that will be less toxic than current approaches, yet facilitate hematopoietic engraftment from haploidentical donors. These aims will not only define the minimum absorbed radiation dose of target tissues required for haploidentical engraftment using novel digital autoradiographic imaging techniques, elucidate the mechanism of cellular damage, but also identify other potential therapeutic interventions to improve leukemia treatment options in future research. This NCI Mentored Research Scientist K01 Award proposal builds on prior results by Dr. Orozco in optimizing radionuclides for anti-CD45 RIT, and is bolstered with a well thought out career development plan, with added translation and clinical training, with the guidance from his career advisory committee. The Committee is comprised of nationally prominent investigators such as: Dr. Orozco's mentor, Dr. Oliver W. Press, a physician scientist and undisputed leader in the field of radioimmunotherapy for hematologic malignancies; Dr. Jose Lopez, an established Latino physician scientist who is Chief Scientific Officer at the Bloodworks Northwest, formerly Puget Sound Blood Center; Dr. Janine McCune, Professor in the Department of Pharmacy at the UW, who is known for her research on pharmacokinetics and pharmacodynamics of anticancer agents; and Dr. Ed Clark, Professor in the Department of Immunology at the UW, who is known for his research on B lymphocyte and dendritic cell regulation. The Fred Hutchinson Cancer Research Center, the University of Washington, and the Seattle Cancer Care Alliance have a wealth of resources, clinical and basic science investigators, making Seattle an ideal location from which to embark on a career to become a successful independent investigator at the bench optimizing antibody based therapy options for hematologic malignancies, and opportunities to improve outcomes in haploidentical HCT.